Diamond is the hardest substance known and has been used as a granular abradant. As a means for its manufacture, the ultra-high pressure method has been in use from a long time ago. In recent years, vapor phase methods (hereinafter referred to collectively as "CVD") of synthesizing diamond have been proposed. In CVD, though membrane diamond is produced mostly, granular diamond can be produced under certain conditions of production.
As one of the methods for the production of granular diamond there is a method which comprises preparing as cores minute diamond granules produced by the ultra-high pressure method and causing CVD diamond to be deposited on the periphery of the cores (Japanese Patent Publication SHO 48-32519 and Japanese Patent Public L Disclosure SHO 47-38794).
In the conventional method, the diamond granules used as cores are expensive and these diamond granules are obtained either by crushing large diamond grains or by classifying diamond dust which is produced during the cutting of diamond. These diamond granules are not easily obtained in a shape closely approximating a sphere because of their sharp edges, for example. It has been difficult, therefore, to produce diamond granules having a shape closely approximating a sphere by causing the core diamond described above to grow.
Where a granular abradant is used particularly for polishing the surface of a given object to high smoothness, the individual granules thereof are desired to be those of a shape closely approximating a sphere. Preferably these granules are mainly in a polycrystalline structure such that they reveal new crystal faces incessantly while they are being used for the abrasion.
Further single crystals of diamond are molded together with a substance as a matrix to produce a grindstone. In this case, the single crystals are not retained with ample fastness in the matrix substance. Generally, the fastness of the retention of the single crystals in the matrix is improved by having the surface of these single crystals coated in advance with a layer capable of promoting intimate contact with the matrix or having the single crystals subjected in advance to a surface treatment. For example, the diamond granules for resinoids are coated with nickel (50 to 60% by weight) by the electroless plating method to enhance their ability to make intimate contact with the matrix and improve their anchoring effect within the matrix.
Treatment of this type adds to the production cost of abrasive granules and grindstones. Moreover, since the coating is generously applied, it is difficult to maintain the desirable cutting ability.
An object of this invention is to provide composite diamond granules which are nearly spherical crystalline particles capable of retaining a surface particularly suitable for abrasion, exhibiting a highly desirable abrading ability as abrasive granules, and, on being molded together with a matrix substance to produce a grindstone, manifesting an ability to maintain intimate contact with the matrix substance and, therefore, obviate the necessity for any surface treatment.